Steel sheet pile walls are constructed by driving steel sheets into a slope or excavation. Their most common use is within temporary deep excavations. They are considered to be most economical where retention of higher earth pressures of soft soils is required.
They have an important advantage in that they can be driven to depths below the excavation bottom and so provide a control to heaving in soft clays or piping in saturated sands. This is not possible with the Soldier pile which is also a more permeable structure. However sheet piles are more costly and less adaptable to hard driving conditions particularly where boulders or irregular rock surfaces occur.
Easy driving conditions are experienced in clays, sands, and clay-sand mixture due to the comparatively small displacement of soil. However they may permit large movements in weak soils and also effective de-watering is often required since they do not provide a watertight boundary. Seepage commonly occurs through the interlocks and this can be sufficient enough to cause consolidation of organic soils and soft silty clays, (compressible materials). For sandy soils raveling will not occur if the interlocks are tight, but driving sheet piles into loose sand can cause subsidence.
There are 3 main types of sheet pile walls.
Cantilever sheet pile walls are mainly used for temporary excavations of moderate depth. Because of the large earth pressures and deflections that may develop they are rarely used to retain excavations greater than a depth of
The required penetration depth is high because the support is totally derived from the passive pressure exerted on the embedded portion of the pile. The deflections at the head of the wall are high and it is not advisable to use cantilever walls when services or foundations are within the active zone since movement in the retained soil may cause damage.
One main advantage of cantilever sheet pile walls is that they can be pulled and reused.
Following the installation of steel sheet piles a small excavation is made along the wall and the first row of anchors is installed. The trench is only made wide enough for the anchor installation machine and the excavation-anchor sequence is repeated until the bottom is reached.
Well constructed anchor walls undergo less lateral deflection than braced walls and so provide a better control of backslope subsidence. Anchor installation only requires a small excavation to allow equipment access. However for braced wall installation there is often a requirement to excavate below the level of support.
Anchored walls are always pre stressed which essentially removes the slack from the system. The anchors will maintain their load throughout the excavation sequence unless creep occurs. The anchors also place the entire soil mass between the anchors and the wall in compression, thus creating a very large gravity wall.
There are two causes of loss of subsidence for anchor systems:
· caving of the anchor holes prior to grouting
· flow of cohesion less material into the excavation through wall openings made for anchor installation
Other disadvantages include possible downward movement of the wall due to the vertical component of the anchor forces.
Propped walls may have one of more levels of prop in the upper part of the wall. They can be designed to have fixed or free earth support at the bottom and derive their stability from the props. They are common in cofferdams.
For propped walls in the free earth condition the penetration of the piles should be such that the passive pressure in front of the piles will resist forward movement of the toes of the piles but will not prevent rotation. The piles are supported by ties at the top of the wall and the soil at the base of the wall.
In fixed earth conditions further penetration of the pile is required to ensure that not only the passive pressures in front of the wall resist forward movement but also that the rotation of the toe is restrained by the passive pressures located near the toe at the rear of the wall